Normally, a rolling bearing for supporting the spindle is incorporated into the spindle of the machine tool exemplified above and generally involves using an angular contact ball bearing, a cylindrical roller bearing, or the like in combination. Herein, accuracy of finishing and productivity of the machine tool depend largely on a rotational speed of the spindle, and the rotations of the spindle must be speeded up in order to enhance the productivity. If the rolling bearing is employed under the high-speed rotations, however, heat generation of the bearing remarkably rises, and a contact pressure between a rolling element and inner/outer rings increases due to centrifugal force. Hence, a condition of using the spindle gets remarkably deteriorated, and resultantly there increases a possibility of damaging the bearing as typified by abrasion, seizing, or the like. Further, the heat generation rises due to the high-speed rotations, and hence there is a possibility that thermal deformation of the machine tool might occur, which also affects the accuracy of finishing.
For preventing the critical inconvenience from occurring in the bearing and for avoiding the decrease in the accuracy of finishing due to the thermal deformation of the whole machine tool, the heat generation in the rolling bearing for supporting the spindle must be restrained to the greatest possible degree by selecting a proper lubrication system under the high-speed rotations. By contrast, the lubrication of the rolling bearing for supporting the spindle of the machine tool rotating at the high speed acquires a cooling effect that accompanies the supply of the lubricating oil, and therefore involves adopting an oil-air lubrication method, a nozzle jet lubrication method and an under-race lubrication method (refer to Japanese Patent Laid-Open Publication No. 2003-278773).